To phosphorus oxychloride (0.35 ml, 3.82 mmol) cooled at 0° C. under argon, was added dropwise anhydrous DMF (0.34 ml, 4.46 mmol) and the mixture was stirred for 5 min forming a precipitate. 1.5 ml of DMF were added and stirring was continued at 0° C. for 15 min. To this mixture was added dropwise a solution of the compound of step 2 (0.476 g, 1.27 mmol) in 5 ml of DMF, and the mixture was heated at 60° C. for 1.5 h and then stirred at room temperature overnight. A solution of sodium acetate (0.837 g) in 6.5 ml water was added, the mixture was heated at 50° C. for 1 h, and evaporated to dryness under reduced pressure. The residue was dissolved in EA, the solution was washed with a saturated solution of sodium hydrogencarbonate, water and brine, dried over sodium sulfate, filtered and evaporated to dryness under reduced pressure. The residue was purified by chromatography on silica gel (cyclohexane/EA, 100:0, then 90:10) to give 0.297 g of the title compound.
